Film support system

ABSTRACT

Film is transferred from one storage reel to another by applying torques to the reels in opposite rotational directions. The ratio of the torques applied to the reels is adjustable such that the sum of the torques remains constant. The excitation windings of the reel driving motors are connected to the outputs of a differential amplifier that maintains the sum of the excitation currents constant. The torques applied to the reels are adjusted by a control signal applied to one input of the differential amplifier so as to maintain approximately constant film transport speed. The control signal is representative of the deviation of the sum of the angular velocities of the reels from an adjustable constant value.

United States Patent 1191 Mattes [54] FILM SUPPORT SYSTEM [75] Inventor:Lyle T. Mattes, Torrance, Calif.

-[73] Assignee: Cutler-Hammer, Inc., Milwaukee,

Wis.

[22] Filed: Sept. 21, 1970 [211 Appl. No.: 74,059

[52] US. Cl ..3l8/7 [51] Int. Cl. ..B65h 77/00 [58] Field of Search..3l8/7, 6

[ 56] References Cited UNITED STATES PATENTS 3,079,538 2/1963 Yamashita..318/7 3,283,228 11/1966 Asseo 3,501,682 3/1970 Jacoby 12/1965 Dingler..3l8/7 OTHER PUBLlCATlONS IBM Technical Bulletin Vol. 13 No. 1 June,1970 p. l 13, Reel Feed Tape Drive, F. A. Schultz et. al.

[ Feb. 6, 1973 IBM Technical Bulletin Vol. 2 No. 6 April, 1960 p. 23,Control Tension Gradient Rewind System H. E. Van Winkle.

Primary ExaminerBernard A. Gilheany Assistant Examiner-W. E. Duncanson,Jr. Attorney-Christie, Parker & Hale [57] ABSTRACT Film is transferredfrom one storage reel to another by applying torques to the reels inopposite rotational directions. The ratio of the torques applied to thereels is adjustable such that the sum of the torques remains constant.The excitation windings of the reel driving motors are connected to theoutputs of a differential amplifier that maintains the sum of theexcitation currents constant. The torques applied to the reels areadjusted by a control signal applied to one input of the differentialamplifier so as to maintain approximately constant film transport speed.The control signal is representative of the deviation of the sum of theangular velocities of the reels from an adjustable constant value.

19 Claims, 4 Drawing Figures PATENTEDFEB s 1913 SHEET 2 BF 2 FILMSUPPORT SYSTEM BACKGROUND OF THE INVENTION This invention relates to thetransport of a length of flexible ribbon such as film between storagereels and, more particularly, to a transport system that maintainssubstantially constant ribbon tension and/or transport speed during thetransfer of the entire length of the ribbon from one reel to the other.

In a number of film transport systems, a requirement for substantiallyconstant film tension is imposed. If the tension on thefilm is toogreat, the film may break or become damaged and if the tension on thefilm is too small, thefilm may sag or deviate from the desired film pathbetween reels. The problem of sagging becomes more serious as the widthof the film is increased because of the additional weight. At constanttransport speed, the tension on the film equals the torque exerted onthe takeup reel by the reel drive motor divided by the radius of theouter layer of film on the takeup reel. Thus, to maintain a constanttension on the film, the torque applied to the takeup reel must beadjusted as the footage of the film on the takeupreel changes in thecourse of the film transfer from one reel to the other.

Conventionally, film tension in a transport system is controlled by aservo including a spring-loaded tension arm disposed in the film path.The movement of the tension arm, which represents the tension exerted onthe film, is sensed and the takeup reel drive motor is adjustedresponsive to the sensed movement of the tension arm so as to maintain asubstantially constant film tension. In some cases, it has been foundnecessary to apply a drag to the supply reel by means ofa brake or amotor in order to provide the required amount of film tension. Thetension arm is relatively unreliable, occupies appreciable space alongthe film path, and necessitates the provision of additional film guidingelements.

' In a number of film transport systems, a requirement for substantiallyconstant transport speed is imposed.

' For example, a photo interpretation system should have a relativelyconstant, slow film transport speed while the operator is visuallysearching thefilm for a particular item. Jerky or uneven film transportis bothersome to the operator who is viewing each item as it passes. Inanalogous fashion to film tension, the film transport speed is-equal tothe angular velocity of the takeup reel multiplied by the radius of theouter layer of the film on the takeupreel. Conventionally, the transportspeed is controlled by a servo that adjusts the angular velocity of thetakeup reel as the radius of the outer layer of film changes in thecourse of the film transfer between reels. The film passes over acapstan in the film path between reels and the capstan'is coupled to atachometer. The takeup reel drive motor is adjusted responsive to theoutput of the tachometer to maintain constant transport speed. Onedisadvantage with theconventional transport speed control servo is therequirement for the capstan, which is not always available or practical.For one thing, the film must be wrapped around a substantial portion ofthe capstans circumference, thereby often making the film pathmoretortuous than it would otherwise have to be.

Moreover, the described servos for controlling film tension andtransport speed are mutually exclusive of one another. If transportspeed is controlled, then film tension varies uncontrolled, and viceversa.

SUMMARY OF THE INVENTION The invention is based upon the assumption thatthe sum of the radii of the outer layers of film on both storage reelsremains approximately constant during transfer of the entire length offilm from one reel to the other. The validity of this assumption dependsupon the ratio of the maximum radius of the outer layer of film on onereel to the radius of the reel hub. It has been found that theassumption is useful for small ratios.

According to one aspect of the invention, the film tension in atransport system is maintained substantially constant by applying to thestorage reels oppositely directed adjustable torques whose sum remainsconstant. Preferably, the torques are applied to the respec tive reelsby motors. An electric current is applied to the excitation windings ofthe motors by control circuitry that maintains the sum of the excitationcurrents constant. The control circuitry could comprise a differentialamplifier, the excitation windings of the motors being connectedrespectively to the outputs of the differential amplifier. One input ofthe differential amplifiercould be held at a constant referencepotential and the other input could be adjusted in accordance with thedesired ratio of the torques. to-be applied to the reels.

A featureof the above-described transport system for maintainingconstant film tension is the automatic adjustment of the ratio of thetorques applied to thereels to maintain a constant transport speed withthe constant film tension; Specifically, a control signal is generatedthat represents the deviation of the actual film transport speed from aselected value. The control signal is applied to the adjusted input ofthe differential amplifier.

According to another aspect of the invention, a constant film transportspeed is approximated by controlling the transfer of film from one reelto another so the sum of the angular velocities of the reels remainsconstant. A control signal is generated that is representativeof thedeviation of the sum of the actual angular velocities of the reels froma selected value. This control'signal could be applied to the motor forthe takeup reel only, with no consideration being given to the controlof film tension, or it could be applied to the adjusted input of thedifferential amplifier discussed above. In the latter case, constantfilm tension is also maintained.

BRIEF DESCRIPTION OF THE DRAWINGS The features of the specificembodiments of the best I mode contemplated for carrying out theinvention are illustrated in the drawings, in which:

FIG. 1 is a schematic diagram of a film transport system for maintainingconstant film tension and speed;

FIG. 2 is a graph depicting the motor excitation currents for the reelsof the transport system shown in FIG. 1 and the tension exerted on' thefilm during film transfer from one reel to the other;

FIG. 3 is a schematic diagram of a film transport system forapproximating a constant film transport speed; and

FIG. 4 is a graph depicting the film transport speed of the transportsystem shown in FIG. 3 during film transfer from one reel to the other.

DETAILED DESCRIPTION OF THE SPECIFIC EMBODIMENT In FIG. 1, rotatablysupported film storage reels 1 and 2 are shown. A length of film 3 iswound around reels 1 and 2 and is directed therebetween by guide rollers4, 5, 6, and 7. A device 8 for utilizing the film is disposed in thefilm path between rollers 5 and 6. By way of example, utilization device8 could be a light table of the type shown in the copending applicationof Thomas F. Adams, et al., Ser. No. 821,914, filed May 5, 1969, whichis assigned to the assignee of the present application.

A drive motor is mechanically coupled to reel 1 by a linkage representedwith a dashed line 16, and a drive motor 17 is mechanically coupled toreel 2 by a linkage represented with a dashed line 18. Motors 15 and 17apply torque to reels 1 and 2 in opposite rotational directions totransfer film 3 from one reel to the other. The torque applied to reel 1by motor 15 is in the rotational direction in which film 3 winds ontoreel 1, as indicated by the arrow at reel 1. Similarly, the torqueapplied to reel 2 by motor 17 is in the rotational direction in whichfilm 3 winds onto reel 2, as indicated by the arrow at reel 2. When thefilm is wound on reels 1 and 2 as shown in FIG. 1, motor 15 applies acounterclockwise torque to reel 1 and motor 17 applies a clockwisetorque to reel 2. If film 3 were wound 'onto reel 2 oppositely from thatshown in FIG. 1, motor 17 would have to apply a counterclockwise torqueto reel 2 in order to wind film 3 onto reel 2. In such case, motors l5and 17 would both apply a counterclockwise torque to the respectivereels, but the torques would be applied in opposite rotationaldirections in the sense that the term is used in the presentspecification, i.e., the torques oppose each other due to the connectionbetween reels provided by film 3.

It has been found that the tension exerted on film 3 can be heldsubstantially constant throughout the transfer of the entire film lengthby maintaining constant the sum of the torques applied to reels 1 and 2by motors l5 and 17, respectively. The tension on film 3 at constanttransport speed can be expressed by the following equation:

l i z z (I) where T is the film tension, 1-, is the torque on reel 1, 1is the torque on reel 2, R is the radius of the outer layer of film onreel 1, and R is the radius of the outer layer of film on reel 2; Byrearranging equation (I), the following equation is developed:

By adding one to both sides of equation (2) and algebraic manipulation,the following equation is derived.

The sum of the radii of the outer layers of film on both reels, R, Rremains substantially constant as long as the ratio of the maximumradius of the outer layer of film on the reel to the radius of the reelhub is not too large. For example, a ratio of 5 to l causes R R to varyby only 20 percent. Therefore, ifit is assumed R, R is constant,according to equations (1) and (3) the film tension can be made constantby holding constant the sum of the torques applied to the reels, 1', +1

To hold constant the sum of the torques applied to reels 1 and 2, theexcitation windings of motors l5 and 17 are energized by a controlcircuit 20. The sum of the excitation currents applied to the windingsof motors l5 and 17 by control circuit 20 remains constant, although theratio of these excitation currents is adjustable. Control circuit 20includes transistors 21 and 22, which are connected to form adifferential amplifier. A source of positive bias potential V isconnected to the collector of transistor 21 by the excitation winding ofmotor 15 and to the collector of transistor 22 by the excitation windingof motor 17. The emitter of transistor 21 is connected by a resistor 23to one terminal of a variable resistor 24 whose other terminal isconnected to ground. The emitter of transistor 22 is connected to theungrounded terminal of variable resistor 24 by a resistor 25. Variableresistor 24 permits the sum of the excitation currents of motors l5 and17 to be changed to a new value. The base of transistor 21 is driven bya transistor 27, which functions as an emitter follower. The emitter oftransistor 27 is connected directly to the base of transistor 21 and toground by a resistor 29. The base of transistor 27 isconnected directlyto the collector of a transistor 26 and to a source of positive biaspotential V by a resistor 30. Source V is at a lower potential thansource V The base of transistor 22 is driven by a transistor 41, whichfunctions as an emitter follower. The emitter of transistor 41 isconnected directly to the base of transistor 22 and to ground by aresistor 46. The base of transistor 41 is connected directly to thecollector of a transistor 40 and to source V by a resistor 47.

Transistors 26 and 40 are connected to form a differential amplifier.The emitter of transistor 26 is connected to ground by resistors 31 and32 in series. The emitter of transistor 40 is connected to the junctionof resistors 31 and 32 by a resistor 48. It is advantageous tosubstitute a transistor that draws constant current for resistor 32. Thebase of transistor 26 is connected to a reference source of positivebias potential V by a resistor 28. Reference source V is at a lowerpotential than source V An adjustable source of positive bias potentialis provided by a potentiometer 42. The end terminals of potentiometer 42are connected between source V and ground. The movable contact ofpotentiometer 42 is connected to the base of transistor 40 through aresistor 43. The base of transistor 40 is protected against largedeviations from the reference potential by diodes 44 and 45, which areconnected between bias source V and the base of transistor 40.Therefore, the potential at the base of transistor 40 is not permittedto deviate from the reference potential by more than the small voltagedrop across diode 44 or diode 45.

When the movable contact of potentiometer 42 is adjusted to causeexcitation currents in motors l5 and 17 proportional to the film radiion the associated reels 1 and 2, the resulting forces applied to thefilm are equal and the film is at a standstill. The relatively highratio between the magnitude of the unbalanced torques produced by motorsl5 and 17 to the unbalanced input to the differential amplifier causesthis adjustment to occur when the voltage at the movable contact ofpotentiometer 42 is nearly equal to the reference potential of source VWhen the movable contact of potentiometer 42 is adjusted so itspotential is above the potential resulting in film standstill, theexcitation current applied to motor 17 decreases and the excitationcurrent applied to motor increases. As a result, the film is transportedfrom reel 2 to reel 1, motor 15 is driving, and motor 17 is dragging.Similarly, when the movable contact of potentiometer 42 is adjusted soits potential is below that resulting, in film standstill, theexcitation current applied to motor 17 increases and the excitationcurrent applied to motor 15 decreases. As a result, the film istransported from reel 1 to reel 2, motor 17 is driving, and motor 15 isdragging.

The transport system of FIG. 1 can be employed to control automaticallythe transport speed of film 3. A tachometer 60 ismechanically coupled toguide roller 7 by a linkage represented with a dashed line 61.Tachometer 60 generates a voltage proportional to the angular velocityof roller 7, which is in turn proportional to the transport speed offilm 3. Tachometer 60, a resistor 62, and a switch 63 are connected inseries between source V and the base of transistor 40. When switch 63-isclosed, a. constant transport speed servo is formed. The desiredtransport speed is selected by adjusting potentiometer 42. The servodrives control circuit 20 so as to hold the potential at the base oftransistor 40 nearly equal to the reference potential of source V As aresult, the actual transport speed is maintained at the selected value,which can be varied at will in either direction from zero.

I stant. In the latter case, the film tension would not be In FIG." 2,curves 65,66, 67, and 68 represent the 7 operation of thetransportsystem of FIG. 1 graphically.

Curve 65 represents the excitation current in amperes of motor 15 (alsomotor torque), as a function of the percentage of .the total length offilm stored on reel 2.

Curve 66 represents the excitation current-of motor 17 in amperes (alsomotor torque) as a function of the percentage of film stored on reel 2.Curve 67, which is the sum of curves 65 and 66, is constant along theentire abscissa of the graph. Curve 68 represents the tension in ouncesexerted on film 3 as a function of the percentage of film on reel 2-,assuming a reel witha one inch hub radius and a maximum outer film layerradius of five inches. As depicted in the graph, the film tensionremains substantially constant during the transfer of the entire lengthof the film from one reel to the other, varying by only about 20percent. If a constant torque were applied to one reel, the film tensionwould vary over a 5 to 1 range during the transfer of the entire lengthoffilm.

. In FIG. 3, storage reels 70 and 71 are rotatably supported. A lengthof film 72 is wound on reels 70 and 71 and is directed therebetween byguide rollers 73, 74, 75, and 76. A utilization device 77 is disposedbetween rollers 74 and 75. Reel drive apparatus. 78 is represented as ablock in FIG. 3. Apparatus 78 could be the same as the reel driveapparatus of FIG. 1, or it could be conventional reel drive apparatus,e.g., a single motor driving only one reel. In the former case, theinput to apparatus 78 would be the base of transistor 40 and the filmtension would be held substantially con held substantially constant. Inany case, a source of positive potential V provides a reference toapparatus 78 for film speed control.

A tachometer 79 is mechanically coupled to reel 71 by a linkagerepresented with a dashed line 80. A tachometer 81 is mechanicallycoupled to reel 70 by a linkage represented with a dashed line 82.Tachometer 79 and a resistor 83 are connected in series between a sourceof positive bias potential V, and the input of apparatus 78. Positivebias potential V, is the same as the reference potential applied to theother input of apparatus 78. Tachometer 81 and a resistor 84 are alsoconnected between source V and the input of apparatus 78..The endterminals of a potentiometer 85 are connected between ground and asource of positive bias potential V which is larger than the potentialof V4. The movable contact of potentiometer 85 is connected by aresistor 86 to the input of apparatus 78.

The system of FIG. 3 approximates a constant film transport speed on thebasis of the angular velocities of reels 70 and 71. Specifically, thesystem drives reels 70 and 71 so the sum of their angular velocitiesremains constant. It has been discovered that when this condition ismet, the film transport speed remains approxi-.

mately constant.

In FIG. 4, the graph represents the percentage of the age of the totallength of film that is stored on reel 2, as-

suming a reel with a 1 inch hub radius and a maximum outer film layerradius of 5 inches. The recognition that the film transport speedremains approximately constant when the sum of the angular velocities ofthe reels is held constant is also based upon the fact that the sum ofthe radii of the outer layers of film on both reels remainssubstantially constant when the ratio of the maximum radius of the outerlayer of film on a reel to the radius of the reel hub is not too large.However, a comparison of FIGS. 2 and 4 shows that the transport speed inthe system of FIG. 2 is not held as constant as the film tension in thesystem of FIG. 1.

The transport speed of film 72 is adjusted by positioning the movablecontact of potentiometer 85. When the movable contact of potentiometer85 is precisely positioned so its potential equals the potential ofsource V film 72 is at rest. As the movable contact of potentiometer 85is moved away from this position, a control signal is generated at theinput of apparatus 78 which drives reels 70 and 71. As a result, voltagedrops are generated across tachometers 79 and 81 that offset the changein potential at the movable contact of approximately at the potential ofbias source V The invention can also be used to transport other types offlexible ribbon between storage reels such as magnetic tape.

What is claimed is:

l. A transport system for a length of flexible ribbon comprising:

first and second rotatably supported storage reels;

means for guiding ribbon between the reels;

means for applying torques to the reels in opposite rotationaldirections to transfer the ribbon from one reel to the other;

and means for adjusting the torques applied to the reels such that theirsum remains constant.

2. The transport system of claim 1, in which the means for adjusting thetorques comprises means for generating a control signal representativeof the deviation of the actual ribbon transport speed from a selectedtransport speed, and means responsive to the control signal foradjusting the ratio of the torques applied to the first and second reelsso as to reduce the deviation.

3. The system of claim 2, in which the guiding means includes a rolleracross which the ribbon passes in nonslipping contact between reels; andthe means for generating a control signal comprises a tachometer coupledto the roller to produce a signal representative of the angular velocitythereof, a source of bias signals representative of a selected transportspeed,.and means for differentially combining the output of thetachometer and the output of the bias source.

4. The transport system of claim 3, in which the source of bias signalsis adjustable. I

5. The transport system of claim 2, in which the means for generating acontrol signal comprises a first tachometer coupled to the first reel toproduce a signal representative of the angular velocity thereof, asecond tachometer coupled to the second reel to produce a signalrepresentative of the angular velocity thereof, a source of bias signalsrepresentative of a selected transport speed, and means fordifferentially combining the outputs of the first and second tachometerswith the output of the bias source.

6. The transport system of claim 5, in which the source of bias signalsis adjustable.

7. The transport system of claim 1, in which the means for adjusting thetorques applied to the reels comprises means for generating a controlsignal representative of the deviation of the sum of the angularvelocities of the reels from a selected value, and means responsive tothe control signal for adjusting the ratio of the torques applied to thefirst and second reels so as to reduce the deviation.

8. The transport system of claim 1, in which the means for applyingtorques to the reels are first and second motors each having anexcitation winding, the torque produced by each motor being proportionalto the current passing through its excitation winding; and the means foradjusting the torques comprises a source of electrical energy, means forapplying excitation current from the source to the windings of themotors such that the sum of the individual excitation currents remainsconstant, and means for adjusting the ratio of the individual excitationcurrents applied to the windings of the motors.

9'. The transport system of claim 8, in which the current applying meanscomprises a differential amplifier, circuitry for connecting the sourceto the differential amplifier as bias therefor, circuitry for connectingthe windings of the motors respectively to the outputs of thedifferential amplifier, and a constant bias source connected to oneinput of the differential amplifier; and the means for adjusting theratio of the currents comprises a manually adjustable bias sourceconnected to the other input of the differential amplifier.

10. A transport system for a length of flexible ribbon comprising:

first and second rotatably supported storage reels;

means for guiding ribbon between the reels;

means for driving the first reel to transfer ribbon from the second reelto the first reel;

and means for adjusting the reel-driving means such that the sum of theangular velocities of the reels remains constant.

11. The transport system of claim 10, in which the adjusting meanscomprises a first tachometer coupled to the first reel to produce afirst signal representative of the angular velocity thereof, a secondtachometer coupled to the second reel to produce a second signalrepresentative of the angular velocity thereof, means for generating acontrol signal representative of the deviation of the sum of the firstand second signals from a selected constant, and means responsive to thecontrol signal for adjusting the reel-driving means so as to reduce thedeviation.

12. The transport system of claim 11, in which the control signalgenerating means comprises a variable source of bias signals and meansfor differentially combining the first and second signals with theoutput of the source of bias signals.

13. The transport system of claim 12, in which the reel-driving meanscomprises first and second motors coupled respectively to the first andsecond reels, each motor having an excitation winding; means forapplying current to each excitation winding such that the sum of 'theindividual currents applied to the excitation windings is constant, andmeans responsive to the control signal for adjusting the ratio of theindividual currents applied to the excitation windings to reduce thedeviation between the sum of the angular velocities of the reels and theselected constant.

14. A transport system comprising:

a first rotatably supported storage reel having a hub;

a second rotatably supported storage reel having a hub;

a length of flexible ribbon wound on the hubs of the first and secondreels and extending therebetween, the ratio of the maximum radius of theouter layer of the ribbon on each reel to the radius of the hub of suchreel being sufficiently small that the sum of the radii of the outerlayers of the ribbon on both reels remains substantially constant; 7

means for applying to the first reel a variable driving torque in adirection to unwind the ribbon from the second reel and wind it onto thefirst reel; and

means for applying to the second reel a variable drag that opposes theunwinding of the ribbon from the second reel, the sum of the variabledrag on the second reel and the variable driving torque on the firstreel being constant.

15. The transport system of claim 14, in which the driving torqueapplied to the first reel is controlled to maintain a constant transportspeed.

16. The transport system of claim 14, in which the driving torqueapplied to the first reel is controlled to maintain the sum of theangular velocities of the reels substantially constant.

17. The transport system of claim 14, in which the means for applying avariable driving torque to the first reel comprises a first motor havingan excitation winding and means for applying an excitation current tothe winding of the first motor; the means for applying a variable dragto the second reel comprises a second motor having an excitation windingand means for applying an excitation current to the winding of thesecond motor, the sum of the individual excitation currents of the firstand second motors being constant; and means for adjusting the ratio ofthe individual excitation currents of the first and second motors.

. 18. The transportsystem of claim 1, in which the first and secondstorage reels each have a hub, the system additionally comprising anelongated flexible ribbon wound on the hubs of the first and secondreels and guided therebetween by the guiding means, the length of theribbon relative to the radius of the hub of each reel being sufficientlysmall that the sum of the radii of the outer layers of the ribbon onboth reels remains substantially constant during transport of the entirelength of the ribbon from one reel to the other.

19. The transport system of claim 10, in which the first and secondreels each have a hub, the transport system additionally comprising anelongated flexible ribbon wound on the hubs of the first and secondreels and guided therebetween by the guiding means, the ratio of themaximum radius of the outer layer of the ribbon on each reel to theradius of the hub of such reel being sufficiently small that the sum ofthe radii of the outer layers of ribbon on both reels remainssubstantially constant during transport of the entire length oftheribbon from one reel to the other.

1. A transport system for a length of flexible ribbon comprising: firstand second rotatably supported storage reels; means for guiding ribbonbetween the reels; means for applying torques to the reels in oppositerotational directions to transfer the ribbon from one reel to the other;and means for adjusting the torques applied to the reels such that theirsum remains constant.
 1. A transport system for a length of flexibleribbon comprising: first and second rotatably supported storage reels;means for guiding ribbon between the reels; means for applying torquesto the reels in opposite rotational directions to transfer the ribbonfrom one reel to the other; and means for adjusting the torques appliedto the reels such that their sum remains constant.
 2. The transportsystem of claim 1, in which the means for adjusting the torquescomprises means for generating a control signal representative of thedeviation of the actual ribbon transport speed from a selected transportspeed, and means responsive to the control signal for adjusting theratio of the torques applied to the first and second reels so as toreduce the deviation.
 3. The system of claim 2, in which the guidingmeans includes a roller across which the ribbon passes in non-slippingcontact between reels; and the means for generating a control signalcomprises a tachometer coupled to the roller to produce a signalrepresentative of the angular velocity thereof, a source of bias signalsrepresentative of a selected transport speed, and means fordifferentially combining the output of the tachometer and the output ofthe bias source.
 4. The transport system of claim 3, in which the sourceof bias signals is adjustable.
 5. The transport system of claim 2, inwhich the means for generating a control signal comprises a firsttachometer coupled to the first reel to produce a signal representativeof the angular velocity thereof, a second tachometer coupled to thesecond reel to produce a signal representative of the angular velocitythereof, a source of bias signals representative of a selected transportspeed, and means for differentially combining the outputs of the firstand second tachometers with the output of the bias source.
 6. Thetransport system of claim 5, in which the source of bias signals isadjustable.
 7. The transport system of claim 1, in which the means foradjusting the torques applied to the reels comprises means forgenerating a control signal representative of the deviation of the sumof the angular velocities of the reels from a selected value, and meansresponsive to the control signal for adjusting the ratio of the torquesapplied to the first and second reels so as to reduce the deviation. 8.The transport system of claim 1, in which the means for applying torquesto the reels are first and second motors each having an excitationwinding, the torque produced by each motor being proportional to thecurrent passing through its excitation winding; and the means foradjusting the torques comprises a source of electrical energy, means forapplying excitation current from the source to the windings of themotors such that the sum of the individual excitation currents remainsconstant, and means for adjusting the ratio of the individual excitationcurrents applied to the windings of the motors.
 9. The transport systemof claim 8, in which the current applying means comprises a differentialamplifier, circuitry for connecting the source to the differentialamplifier as bias therefor, circuitry for connecting the windings of themotors respectively to the outputs of the differential amplifier, and aconstant bias source connected to one input of the differentialamplifier; and the means for adjusting the ratio of the currentscomprises a manually adjustable bias source connected to the other inputof the differential amplifier.
 10. A transport system for a length offlexible ribbon comprising: first and second rotatably supported storagereels; means for guiding ribbon between the reels; means for driving thefirst reel to transfer ribbon from the second reel to the first reel;and means for adjusting the reel-driving means such that the sum of theangular velocities of the reels remains constant.
 11. The transportsystem of claim 10, in which the adjusting means comprises a firsttachometer coupled to the first reel to produce a first signalrepresentative of the angular velocity thereof, a second tachometercoupled to the second reel to produce a second signal representative ofthe angular velocity thereof, means for generating a control signalrepresentative of the deviation of the sum of the first and secondsignals from a selected constant, and means responsive to the controlsignal for adjusting the reel-driving means so as to reduce thedeviation.
 12. The transport system of claim 11, in which the controlsignal generating means comprises a variable source of bias signals andmeans for differentially combining the first and second signals with theoutput of the source of bias signals.
 13. The transport system of claim12, in which the reel-driving means comprises first and second motorscoupled respectively to the first and second reels, each motor having anexcitation winding; means for applying current to each excitationwinding such that the sum of the individual currents applied to theexcitation windings is constant, and means responsive to the controlsignal for adjusting the ratio of the individual currents applied to theexcitation windings to reduce the deviation between the sum of theangular velocities of the reels and the selected constant.
 14. Atransport system comprising: a first rotatably supported storage reelhaving a hub; a second rotatably supported storage reel having a hub; alength of flexible ribbon wound on the hubs of the first and secondreels and extending therebetween, the ratio of the maximum radius of theouter layer of the ribbon on each reel to the radius of the hub of suchreel being sufficiently small that the sum of the radii of the outerlayers of the ribbon on both reels remains substantially constant; meansfor applying to the first reel a variable driving torque in a directionto unwind the ribbon from the second reel and wind it onto the firstreel; and means for applying to the second reel a variable drag thatopposes the unwinding of the ribbon from the second reel, the sum of thevariable drag on the second reel and the variable driving torque on thefirst reel being coNstant.
 15. The transport system of claim 14, inwhich the driving torque applied to the first reel is controlled tomaintain a constant transport speed.
 16. The transport system of claim14, in which the driving torque applied to the first reel is controlledto maintain the sum of the angular velocities of the reels substantiallyconstant.
 17. The transport system of claim 14, in which the means forapplying a variable driving torque to the first reel comprises a firstmotor having an excitation winding and means for applying an excitationcurrent to the winding of the first motor; the means for applying avariable drag to the second reel comprises a second motor having anexcitation winding and means for applying an excitation current to thewinding of the second motor, the sum of the individual excitationcurrents of the first and second motors being constant; and means foradjusting the ratio of the individual excitation currents of the firstand second motors.
 18. The transport system of claim 1, in which thefirst and second storage reels each have a hub, the system additionallycomprising an elongated flexible ribbon wound on the hubs of the firstand second reels and guided therebetween by the guiding means, thelength of the ribbon relative to the radius of the hub of each reelbeing sufficiently small that the sum of the radii of the outer layersof the ribbon on both reels remains substantially constant duringtransport of the entire length of the ribbon from one reel to the other.